1. Field of the Invention
The invention relates to a power conversion apparatus, more particularly, to a power conversion apparatus using an over current protection reference signal with variable slope to determine whether or not to activate an over current protection mechanism.
2. Description of Related Art
A main application of a power conversion apparatus is to convert an alternating current (AC) input voltage with high voltage and low stability that is provided by a power company into a direct current (DC) output voltage with low voltage and high stability that is adapted to various electronic devices. Therefore, the power conversion apparatus is widely used in electronic devices such as computers, office automation equipments, industrial control equipments and communication devices, etc.
FIG. 1 is a schematic diagram illustrating a conventional power conversion apparatus 10. Referring to FIG. 1, generally, a pulse width modulation (PWM) signal generator 103 in a PWM controller chip 101 outputs a PWM signal VPWM to switch (i.e. turn on/off) a power switch Q. In this way, an AC voltage of a secondary side S of a transformer T corresponding to an AC input voltage Vin received by the primary side P of the transformer T (the AC voltage is determined by a turn ratio of the primary and the secondary sides P and S of the transformer T) is rectified by a diode D and filtered by a capacitor C, and is converted into a DC output voltage Vout and provided to an electronic device LD for utilization.
Conventionally, to avoid excessive current Ip (i.e. over current) flowing through the primary side P of the transformer T to cause damage/burnout of the power switch Q and/or the electronic device LD, in the power conversion apparatus 10, an over current protection (OCP) unit 105 in the PWM controller chip 101 is used to detect a voltage Vcs on a node N1 between the power switch Q and a resistor Rs, wherein Vcs=Rs×Ip. When the voltage Vcs on the node N1 reaches a predetermined OCP reference voltage VOCP (which is generally a constant DC voltage) received by the OCP unit 105, the OCP unit 105 activates an OCP mechanism to control the PWM signal generator 103 not to output the PWM signal VPWM, so as to turn off the power switch Q to protect the power switch Q and/or the electronic device LD.
However, the power conversion apparatus 10 is generally applied in wide range AC input occasions (for example, the AC input voltage of 90V-264V). Therefore, in case of an idle state that the input power Pin of the power conversion apparatus 10 is equal to an output power Pout thereof (Pin=Pout), since the predetermined OCP reference voltage VOCP received by the OCP unit 105 is a constant DC voltage, energies of over outputs caused by the OCP mechanism activated by the OCP unit 105 are different in case that the power conversion apparatus 10 receives the lowest AC input voltage Vin (90V) and the highest AC input voltage Vin (264V), so that OCP points corresponding to the OCP mechanism activated by the OCP unit 105 are different in case that the power conversion apparatus 10 receives the lowest AC input voltage 90V and the highest AC input voltage 264V.
To effectively resolve the problem that the OCP points corresponding to the OCP mechanism activated by the OCP unit 105 are different in case that the power conversion apparatus 10 receives the lowest AC input voltage 90V and the highest AC input voltage 264V, conventionally, the predetermined OCP reference voltage VOCP of the constant DC voltage received by the OCP unit 105 can be compensated to a compensated OCP reference voltage VOCP′ having a fixed initial voltage VT climbing to a highest voltage VM along a fixed slope SLP, as that shown in FIG. 2. In this way, the difference of the OCP points corresponding to the OCP mechanism activated by the OCP unit 105 in case that the power conversion apparatus 10 receives the lowest AC input voltage 90V and the highest AC input voltage 264V can be effectively reduced.
However, if the slope SLP is too flat, the difference of the OCP points corresponding to the OCP mechanism activated by the OCP unit 105 in case that the power conversion apparatus 10 receives the lowest AC input voltage 90V and the highest AC input voltage 264V cannot be effectively reduced. On the other hand, if the slope SLP is too steep, although the difference of the OCP points corresponding to the OCP mechanism activated by the OCP unit 105 in case that the power conversion apparatus 10 receives the lowest AC input voltage 90V and the highest AC input voltage 264V can be effectively reduced, since the slope SLP is steeped, in case of maintaining the highest voltage VM of the compensated OCP reference voltage VOCP′, the initial voltage VT of the compensated OCP reference voltage VOCP′ has to be pulled down.
In this way, the OCP unit 105 may intermittently activate/trigger the OCP mechanism at an initial stage of booting the electronic device LD when the power conversion apparatus 10 receives the lowest AC input voltage 90V to provide the DC output voltage Vout to the electronic device LD, so that the electronic device LD cannot be successfully booted since now the voltage Vcs of the node N1 is easy to reach the pulled-down initial voltage VT.